The invention relates to a material for electric contacts taking silver tin-oxide as basis, consisting of silver or a mainly silver-containing alloy, tin oxide and other oxides or carbides of tungsten, molybdenum, vanadium, bismuth, titanium, and/or copper. Such material is known from patent WO 89/09478.
Because of their better environmental compatibility and their at least partly more favorable service life, the contact materials with silver tin oxide have begun to replace the previously preferred silver cadmium-oxide materials. However, because of its higher thermal properties, tin-oxide's tendency under arcing effect to form poorly conducting clinker layers on the surface, at constant current, the thermal behavior of silver tin-oxide contacts is unsatisfactory. In order to remedy this unsatisfactory property, to the generally powder-metallurgically produced material are added powdered admixtures that lead to a lower temperature at the contact points. As appropriate admixtures, the patents disclose especially tungsten and molybdenum compounds (DE-A-29 33 338, DE-A- 31 02 067, DE-A-32 32 627, EP-A-0024349). Bismuth and germanium compounds were further disclosed as admixtures (DE-A-31 02 067 and DE-A-32 627). These admixtures help to wet tin-oxide particles, so that the tin oxide remains finely divided in suspension when the contact piece surface melts under the effect of a contact arcing. Beside this positive effect in respect to the thermal behavior under constant current, these admixtures have, however, undesirable secondary effects. The less than satisfactory plastic deformation of the silver tin-oxide contact materials for the improvement of which the tin-oxide powder is subject, by way of example, to a pre-treatment by calcining (DE-A-29 52 128), is further worsened by the admixtures because of their embrittling effect. This applies especially to the bismuth and molybdenum oxides. Another disadvantage, particularly of the tungsten and molybdenum compounds, is that--especially in switching operation under ACI stress (DIN 57660 Section 102)--they contribute to a transfer of material, leading to an accelerated burn-off and thus to a shortened service life.
As disclosed in WO 89/09478, a material for contacts presenting a low welding tendency and the lowest possible contact temperature can be obtained by purposefully producing a structure in which areas containing little or no metal oxide alternate with areas that contain the entire metal oxide component, or the preponderant portion thereof, in minute distribution. For this purpose, a composite powder is produced that contains the preponderant part of the tin oxide and the other oxides and/or carbides, as well as a portion of the silver. This composite powder is mixed, condensed, sintered, and transformed with the remaining silver powder and eventually with the remaining part of the metal oxides. Although a suitable material is obtained in this manner, it is achieved by using a rather costly method.
The patent EP-A 0 369 283 discloses a sintered contact material for low-voltage switch gears used in power engineering, in particular for motor contactors, the composition AgSnO.sub.2 Bi.sub.2 O.sub.3 CuO. This composition is produced by the internal oxidation of an AgSnBiCu alloyed powder that is mixed, compressed and sintered with a lesser amount of bismuth-zirconate and/or bismuth- titanate powder. This process reduces the strength of the AgSnO.sub.2 Bi.sub.2 O.sub.3 CuO particles at the edge of the oxides, so that between the particles is created a silver network that allows for high compressed densities. However, both the manufacture of the alloy powder as well as its internal oxidation are costly and render the method quite expensive.
The object of this invention is to produce a material of the initially stated type, that through the means of oxidic or carbidic admixtures presents a thermal behavior that is as advantageous as the known materials for [electric] contacts without, however, being as brittle.